The present invention relates to a method and arrangement for improving the hydrolysis of a reduction agent in an exhaust gas post treatment system of internal combustion engines that are operated with excess air, in particular for the selective catalytic reduction of NOx in the exhaust gas of such engines.
Limited exhaust gas components, which result during combustion processes, and the permissible emissions of which are continuously being lowered, include, in addition to solid particles, nitrogen oxides. To minimize these exhaust gas components with internal combustion engines operated in motor vehicles, various methods are presently utilized. The reduction of the nitrogen oxides generally occurs with the aid of catalysts, and in oxygen-rich exhaust gas a reduction agent is additionally required in order to increase the selectivity and NOx conversions. These methods have become known by the collective term SCR processes, whereby SCR stands for “selective catalytic reduction”. It has been used for many years in the power plant industry, and in recent times also with internal combustion engines. A detailed illustration of such processes can be found in DE 34 28 232 A1. V2O5-containing mixed oxides, for example in the form of V2O5/WO3/TiO2, can be used as SCR catalysts. Typical V2O5 proportions are between 0.2-3%. Iron and copper-containing zeolites can also be used as active materials for SCR catalytic converters.
In practice, ammonia, or compounds that release ammonia, such as urea or ammonium formate, in solid or dissolved form, are used as reduction agents. To convert one mol nitric oxide, one mol ammonia is required.4NO+4NH3+O2→4N2+6H2O  (1)
If a platinum-containing NO-oxidation catalytic converter is disposed upstream of the SCR catalytic converter for the formation of NO2,2NO+O22NO2  (2)
the SCR reaction can be considerably accelerated, and the low temperature activity can be significantly raised.NO+2NH3+NO2→2N2+3H2O  (3)
Depending upon the design of the catalytic converters, platinum content, and application, the catalytic converters have start-to-react temperatures between 180° C.-330° C. In this connection, start-to-react temperature means that temperature at which 50% of the nitric oxide oxidizes to nitrogen dioxide.
With internal combustion engines operated in vehicles, the nitrogen oxide reduction with the aid of the SCR process is difficult for the reason that changing operating conditions exist that make the quantitative metering of the reduction agent difficult. Although on the one hand as high a conversion of the nitrogen oxides as possible should be achieved, on the other hand care must be taken that there is no emission of unused ammonia. To prevent this, an ammonia-blocking catalytic converter is frequently disposed downstream of the SCR catalytic converter to convert excess ammonia into nitrogen and water vapor. Particular problems are caused by the so-called cold start, during which the exhaust gas emission must be maintained even after starting of the cold engine, and hence a post treatment system that is not yet operationally warm.
To minimize the solid particles, not only in the power plant industry but also with vehicles, either so-called particle separators or particle filters are used. A typical arrangement having particle separators for use in vehicles is described, for example, in EP 1 072 765 A2. Such arrangements differ from those having particle filters in that the diameter of the channels of the particle separator are considerably greater than the diameter of the largest particle that is present, whereas with particle filters the diameter of the filter channels is in the range of the diameter of the particles. As a consequence of this difference, particle filters are subject to becoming dogged, which increases the exhaust gas counter pressure and reduces the engine power. An arrangement and a method using particle filters instead of a particle separator of the aforementioned type can be found in EP 0 341 832 A2. The aforementioned arrangements or methods are characterized in that the oxidation catalytic converter, which is respectively disposed upstream of the particle separator or particle filter, and which is generally a catalytic converter having platinum as the active material, oxidizes the nitric oxide in the exhaust gas with the aid of the also-contained residual oxygen to nitrogen dioxide, which in turn is converted in the particle separator, or the particle filter, with the carbon particles to CO, CO2, N2 and NO. In this way, at relatively low temperatures, a continuous removal of the accumulated solid particles,2NO2+C→2NO+CO2  (4)NO2+C→NO+CO  (5)2C+2NO2→N2+2CO2  (6)
is effected.
Regeneration cycles, as they must expensively be carried out with other arrangements, in order at relatively high temperatures to oxidize the carbon-containing particles at relatively high temperatures with the aid of oxygen, are thereby eliminated.
In order to fulfill the exhaust gas regulations that will be applicable in the future, it is necessary to simultaneously use not only arrangements for reducing nitrogen oxide emissions, but also arrangements for reducing the emission of solid particles. For this purpose, various arrangements and methods have already become known from DE 103 48 799 A1 and EP 1 054 722 B1.
As already mentioned, in the power plant industry ammonia has proven to be advantageous as a reduction agent for the SCR reaction. However, due to its toxicity, with internal combustion engines operated in vehicles ammonia is replaced by harmless ammonia-releasing compounds such as urea or ammonium formate, in solid or aqueous form. The decomposition of these materials, and hence the release of ammonia, is determinative with SCR methods for the usability of the method.
In conjunction with the decomposition of urea ((NH2)2CO) into ammonia (NH3), it is known that this occurs under optimum conditions (temperatures greater than 350° C.) in two stages; according to(NH2)2CO→NH3+HNCO  (7)
there is first effected the thermolysis, or pyrolysis, i.e. the thermal decomposition, of urea. Subsequently, according toHNCO═H2O→NH3+CO2  (8)
there is effected a hydrolysis, in other words, the decomposition of isocyanic acid (HNCO) into ammonia (NH3) and carbon dioxide (CO2).
Since with the use of the commercially available 33% aqueous urea solution known as AdBlue the reduction agent is present in a form dissolved in water, this water must be evaporated prior to and during the actual pyrolysis and hydrolysis.
If the temperatures present with the previous reactions according to (7) and (8) are below 350° C., or if heating is accomplished only slowly, it is know from DE 40 38 054 A1 that essentially solid cyanuric acid results from trimerization of the isocyanic acid formed according to reaction (7) pursuant to

and as a consequence thereof ammelide,Cyanuric acid+NH3→Ammelide+H2O  (10)
Ammeline,Ammelide+NH3→Ammeline+H2O  (11)
and Melamine,Ammeline+NH3→Melamine+H2O  (12)which lead to clogging of the downstream exhaust gas section. Remedial action can, as outlined in the aforementioned DE 40 38 054, be provided by guiding the exhaust gas stream that is laden with the reduction agent over a urea decomposition and hydrolysis catalytic converter. The exhaust gas temperature from which a quantitative hydrolysis is possible can thus be depressed to 160° C. The construction and composition of an appropriate catalytic converter are also described in the aforementioned publication, as are the construction and function of an SCR catalytic converter system that is equipped with a hydrolysis catalytic converter. TiO2 and/or SiO2 and/or Al2O3 and/or zeolites are used as active components for a urea decomposition and hydrolysis catalytic converter. To reduce the size of the catalytic converters, yet to keep the retention time in the catalytic converters constant, the hydrolysis catalytic converters are often operated in a partial exhaust gas stream which is withdrawn from the main stream, as shown in EP 105 200 9 A1. In this connection, it is particularly advantageous to withdraw the partial exhaust gas stream as close to the engine as possible in order to be able to operate the hydrolysis catalytic converter at a high temperature level. With exhaust gas turbocharged internal combustion engines, it is advantageous to already withdraw the partial exhaust gas stream upstream of the turbocharger and to convey it back again downstream of the turbocharger; appropriate arrangements are described in DE 10206028 A1, DE 19855384 A1 and DE 19960976 A1.
If the hydrolysis catalytic converter is overloaded, for example by being operated at too low of a temperature and/or with high reduction agent metered quantities, no quantitative hydrolysis of the isocyanic acid results. This is particularly problematic with the use of aqueous urea solution, since due to the vaporization of the water additional heat is withdrawn from the exhaust gas and the latter is thus cooled off. In such a case, as described above, thermally very stable by-products such as cyanuric acid, ammelide, ammeline and melamine, are formed. These solid materials accumulate on the hydrolysis catalytic converters, the urea injection nozzles, and the pipes and tubing, which can lead to complete blocking of the exhaust gas stream and the failure of the SCR system. The complete decomposition of these materials takes place only above 450° C.-500° C. However, due to their high efficiency, the exhaust gas temperatures of modern diesel engines is normally below 400° C. The result of this is that deposits that might have formed cannot be again removed without auxiliary measures.
One possibility for raising the exhaust gas temperature to an appropriate level is described in DE 3605255 A1, and comprises adjusting engine parameters, such as the beginning of injection, or lowering the fuel/air ratio.
Furthermore, the proportion of uncombusted hydrocarbons and/or carbon monoxide in the exhaust gas can be increased in order to then oxidize, possibly catalytically, these materials, and hence to increase the exhaust gas temperature. Appropriate arrangements and methods are disclosed in DE 102005023398 A1, DE 10323245 A1, and DE 60210528 T2.
In addition to raising the temperature level by exothermic reactions, it is further known from DE 19960976 A1 to electrically heat the hydrolysis catalytic converters to raise their temperature.
All of the aforementioned variations for raising the temperature have in common that they lead to an increase in fuel consumption and hence to a reduction in the efficiency of the internal combustion engines.
It is therefore an object of the present invention, in addition to avoiding a reduction of the efficiency of an internal combustion engine, to ensure the functionality of a hydrolysis catalytic converter, which is operated in a secondary or by-pass stream of an exhaust gas post treatment system, over a great operating parameter range of the internal combustion engine, and furthermore to reliably prevent problematic deposits, such as cyanuric acid, ammelide, ammeline and melamine, from forming in the secondary exhaust gas stream or downstream thereof.